Low voltage integrated circuits are popular for reasons including low power dissipation, which makes them ideal for use as processors in hand-held and portable commercial electronic products. However, higher voltage integrated circuits continue to be required to perform various functions such as communicating with user interfaces. For example, a low voltage integrated circuit processor may operate at 3 V, while a high voltage integrated circuit interface device may operate at 5 V. Therefore, to construct a commercial product, a combination of 3 V circuits and 5 V circuits are required, where the circuits must be capable of communicating with one another. Circuitry for handling this communication is the subject of the present invention.
FIG. 1 shows a conventional input/output circuit according to the prior art. The circuit is externally supplied with an internal operating voltage (VDD). An external terminal 2 is coupled to an internal terminal 4 via a conductor 5. A pull-up circuit 6 is coupled to VDD and conductor 5. In output mode, output enable (oen) is low and output enable inverse (oenb) is high. The internal signal triggers either pull-up circuit 6 or pull-down circuit 10 to drive the desired output signal to external terminal 2. Although an external circuit coupled to terminal 2 may be operating at a higher voltage than the FIG. 1 circuit, generally an output logic 1 translates to the external circuit as a logic 1 and an output logic 0 translates to the external circuit as a logic 0. For example, a 3 V output is interpreted by a 5 V external circuit as a logic 1 because 3 V is above the logic threshold of approximately 2.5 V, and a 0 V output is interpreted by a 5 V external circuit as a logic 0 because both circuits use 0 as a logic 0.
In input mode, oen is high and oenb is low thereby disabling pull-up circuit 6 and pull-down circuit 10. However, when an external input signal on terminal 2 exceeds the circuit's internal operating voltage (VDD), current flows through a diode 8 in pull-up circuit 6. While, diode 8 limits the input voltage to approximately VDD +V.sub.TH (diode), the current flowing through diode 8 may cause excess energy dissipation and may also adversely affect the integrity of the input signal. An example of a voltage difference that would trigger this harmful condition is when the external input signal is 5 V and the internal operating voltage is 3 V. As a result of this condition, the conventional circuit of FIG. 1 cannot be used by a low voltage integrated circuit to receive a high voltage input signal.
The combined use of low voltage circuits and high voltage circuits creates a motivation to design an input/output circuit that efficiently communicates a low voltage output signal to a high voltage external circuit and efficiently communicates a high voltage input signal to a low voltage internal circuit.